The overall goal of this research is to elcuidate the phyisological mechanisms which govern the maturation of a single preovulatory follicle during the primate menstrual cycle. Our work to date has documented that the feedback relationship between ovarian steriods and gonadotropin secretion is the essential component of the hypothalamic-pituitary-ovarian axis by which the presence of a maturing follicle arrests the development of less mature follicles. Further, we have shown that the maturing follicle continues its growth in the presence of gondatropin concentrations which are unable to initiate the maturation of less mature follicles which explains how the maturing follicle is protected from its own inhibitory influences. During the proposed funding interval we will continue our use of the gonadotropin-pulsed macaque model to investigate the nature of the ovarian responses to gondatrophic hormones. Specifically, we will determine whether follicular growth in response to FSH is a threshold dependent phenomenon. In addition, we will investigate the ovarian mechanisms by which the maturing follicle acquires greater sensitivity to FSH, and whether the pathophysiological connection associated with polycystic ovarian disease is a direct consequence of an altered FSH:LH ratio. Lastly, we will utilize magnetic resonance imaging to measure growth rates of individual follicles as a function of gondatropin concentration as well as to study the ovarian microcirulation during follicle growth and atresia. In addition to providing basic information as follicular growth during the primate menstural cycle, it is expected that results from the proposed study will be of use to endocrinologists in the control of certain forms of infertility.